fix builtins.readFile for non regular files like /dev/stdin

src/libutil/posix-source-accessor.cc

@@ -33,6 +33,13 @@ std::filesystem::path PosixSourceAccessor::makeAbsPath(const CanonPath & path)
         : root / path.rel();
 }
 
+std::string PosixSourceAccessor::readFile(const CanonPath & path)
+{
+    // Can't be implemented as a wrapper of the `Sink` based overload
+    // since this needs to support non-regular files (like `/dev/stdin`).
+    return nix::readFile(path.abs());
+}
+
 void PosixSourceAccessor::readFile(
     const CanonPath & path,
     Sink & sink,
@@ -50,6 +57,9 @@ void PosixSourceAccessor::readFile(
     if (fstat(fd.get(), &st) == -1)
         throw SysError("statting file");
 
+    // logger->cout("%s %d", path.c_str(), st.st_size);
+    assert(S_ISREG(st.st_mode) && "sinks are only compatible with regular files since they need to know the size in advance");
+
     sizeCallback(st.st_size);
 
     off_t left = st.st_size;

src/libutil/posix-source-accessor.hh

@@ -25,6 +25,8 @@ struct PosixSourceAccessor : virtual SourceAccessor
      */
     time_t mtime = 0;
 
+    std::string readFile(const CanonPath & path) override;
+
     void readFile(
         const CanonPath & path,
         Sink & sink,

tests/functional/local.mk

@@ -60,6 +60,7 @@ nix_tests = \
   fetchGitVerification.sh \
   flakes/search-root.sh \
   readfile-context.sh \
+  readfile-stdin.sh \
   nix-channel.sh \
   recursive.sh \
   dependencies.sh \